As an example of conventional antenna devices whose directional characteristics are changeable, a technique which is applied to the Yagi-Uda antenna consisting of three elements is known (Patent Literature 1).
The antenna device disclosed in Patent Literature 1 includes one excitation element (described as Driven element in the Figures of patent literature 1) to which a radio frequency signal is fed, and two passive elements (described as Passive elements in the Figures of Patent Literature 1) which are respectively disposed on both sides across the excitation element and to which no radio frequency signal is fed. Further, switches (described as Optoelectronic switches in the FIGS. of Patent Literature 1) to control conduction characteristics by using light are connected at certain midpoints between both end portions of each of the two passive elements, and, for each of the two passive elements, light is applied as a control signal from an individual laser light source to each of the switches.
When switches are open (described as Open in Patent Literature 1), the portion between a central portion of the passive element and end portions of the passive element where the switches are disposed are electrically non-connected to each other, and therefore the antenna functions as a director. In contrast, when switches are closed (described as Closed in Patent Literature 1), the portions between the central portion and the end portions where the switches are disposed are electrically connected with each other, and therefore the antenna functions as a reflector.
The antenna device is further configured so that the conduction and non-conduction of the switches are reversible between the two passive elements by controlling the existence or non-existence of light from each laser light source, thereby making the directional characteristics of the antenna device changeable.
Further, a technique of arranging a plurality of antennas having controllable directional characteristics as above as a unit antenna to configure an array antenna device is known (Non Patent Literature 1).
The array antenna device disclosed in Non Patent Literature 1 is configured in such a way that switches in each passive element (described as a Parasitic strip in Non Patent Literature 1) disposed in each unit antenna are all controlled by a direct current signal in a same way.
In the above-mentioned explanation of background of the invention and in the following explanation of the present invention, the terms “electrical conduction” and “electrical non-conduction” do not necessarily have to mean strict electrical conduction and non-conduction characteristics, respectively, and antenna devices have only to have electrical conduction characteristics and electrical non-conduction characteristics which are required to such an extent that the antenna devices satisfy the performance necessary thereto.